Windows limitation on number of simultaneously opened sockets/connections per machine

Question

Let's say I have Windows 7 with one real network interface and few loopback interfaces. I have IOCP enabled server that accepts connections from clients. I'm trying to simulate as much as possible real client connections to the server.

My client code simply establishes X amount of socket connections (note that client binds to a given interface):

        const Int32 remotePort = 12345;
        const Int32 MaxSockets = 60000;

        Socket[] s = new Socket[MaxSockets];
        IPEndPoint bindEndpoint = new IPEndPoint(IPAddress.Parse(args[0]), 0);
        for (Int32 i = 0; i < MaxSockets; i++)
        {
            s[i] = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            s[i].SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
            s[i].Bind(bindEndpoint);
            s[i].Connect(args[1], remotePort);

            IPEndPoint socketInfo = (IPEndPoint)s[i].LocalEndPoint;
            Console.WriteLine(String.Format("Connected socket {0} {1} : {2}", i, socketInfo.Address, socketInfo.Port));
        }

On a loopback interface I have several IPs that I use for binding. In addition, I also use real interface to bind on. I ran into a problem when amount of opened sockets is around 64K per machine:

Unhandled Exception: System.Net.Sockets.SocketException: An operation on a socket could not be performed because the system lacked sufficient buffer space or because a queue was full

I've tried several helpless things like: - setting MaxUserPort to max value and some other recommended TCPIP settings in the registry. - trying to run two servers on different interfaces (real interfaces and loopback) and using several clients.

Is it a known limitation in Windows or its possible to overcome it somehow?

Thanks for the help!

Solution 1

I have found on some Microsoft page that:

... HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\MaxUserPort registry subkey is defined as the maximum port up to which ports may be allocated for wildcard binds. The value of the MaxUserPort registry entry defines the dynamic port range...

So, if I force the endpoint to use a certain port, e.g.

IPEndPoint bindEndpoint = new IPEndPoint(IPAddress.Parse(args[0]), 54321);

Then I can open more than 64K simultaneous sockets in the system.

Solution 2

In your code example, you are calling Bind(bindEndpoint), but you do not show how bindEndpoint is defined. Check that :

• Your system actually has multiple IP addresses (loopback does not count)
• You are actually setting the IP Address of the endpoint to an IP address (not loopback)
• The binds are being spread across multiple IP addresses

The loopback address does not count because many systems treat it specially for routing and binding purposes. So binding to ports in loopback may be sucking up the ports across all addresses the same as if you were binding to INADDR_ANY (0.0.0.0).